1. Field of Invention
The present invention relates to inorganic pigments. More particularly, the present invention relates to inorganic pigments comprising cobalt and aluminum having a spinel crystalline structure that exhibit improved color and/or weatherability when used in paint and polymer applications and a method of making the same.
2. Description of Related Art
Inorganic pigments having the spinel crystalline structure can be represented by the general formula: AB2O4, where A is a divalent element or combination of divalent elements and B is a trivalent element or combination of trivalent elements. One common example of an inorganic pigment having a spinel crystalline structure is cobalt aluminate blue spinel (basic chemical formula: CoAl2O4), in which substantially all of the cations represented by A in the general formula AB2O4 comprise the element cobalt and substantially all of the cations represented by B in the general formula AB2O4 comprise the element aluminum. Other inorganic pigments comprising cobalt and aluminum having the spinel crystalline structure include, for example: cobalt zinc aluminate blue spinel (basic chemical formula: (Co,Zn)Al2O4); zinc chrome cobalt aluminum spinel (basic chemical formula: (Zn,Co)(Cr,Al)2O4); cobalt chromite blue-green spinel (basic chemical formula: Co(Al,Cr)2O4); and cobalt tin alumina blue spinel (basic chemical formula: CoAl2O4/Co2SnO4).
In many cases inorganic pigments comprising cobalt and aluminum having the spinel crystalline structure will be modified such that the crystalline structure includes predetermined amounts of cations of divalent and/or trivalent elements other than cobalt and aluminum, which modifies the resulting color of the pigment. Suitable modifiers for such inorganic pigments include, for example, any one or a combination of the following elements: chromium, lithium, magnesium, silicon, strontium, tin, titanium and zinc.
Inorganic pigments comprising cobalt and aluminum having a spinel structure most commonly produced via the high temperature calcination of a raw batch of vigorously intermixed powders containing cobalt and aluminum such as, for example, cobalt (II) oxide (basic formula: CoO) powder and aluminum (III) oxide (basic formula: Al2O3) powder. The reaction product formed via high temperature calcination of the raw batch is a substantially homogeneous cystalline matrix of spinel. Mineralizers, which facilitate a reaction without being part of the desired product, are sometimes utilized to aid the reaction. Conventional mineralizers used in inorganic pigments include NaCl, CaCl2, KCl and CaSO4. Often, the calcined product is ground after calcination to improve its pigmentary properties.
Inorganic pigments comprising cobalt and aluminum having the spinel crystalline structure are used in a variety of applications including, for example, paints and polymers. Paints and polymers containing such inorganic pigments are often exposed to ultraviolet (UV) radiation (e.g., from sunlight) and other environmental conditions. The intensity and color of paint films and polymer products containing such inorganic pigments tend to degrade upon extended exposure to the environment, and over time such paint films and polymer products tend to develop an undesirable faded or bleached out appearance.